Estimates of microplanktonic biomass in Antarctic near-surface waters: the effects of wind.

BOLTOVSKOY DEMETRIO; ALDER VIVIANA

POLAR BIOLOGY

SPRINGER

Lugar: Bremerhaven; Año: 1988 vol. 8 p. 397 - 399

0722-4060

A major goal of many biological Antarctic surveys is describing the patterns of horizontal and vertical distribution of planktonic communities, and their subsequent causal interpretation. Due to operational difficulties, these studies are almost invariably based on one or a few closely spaced in time measurements per site. Some of the biases which are known to routinely affect these estimates (e.g., extrusion of the organisms through the net's meshes, dodging of the sampling gear, diel vertical migrations, patchy distribution, etc.) are often evaluated before drawing conclusions; however, the influence of wind-forced horizontal and vertical mixing and its effect on the distribution of plankton have hardly ever been taken into consideration for interpreting the field data retrieved. Yet, this influence has been subject of several studies in both freshwater and marine environments which demonstrated its overwhelming importance for the distributional patterns observed (e.g., Boltovskoy et al. 1984, and references therein; Taggart and Leggett 1987).  Between 22 February and 23 March, 1987, 142 plankton samples were collected in the Drake Passage, Weddell and Bellingshausen Seas (Fig. 1, top) from the Argentine icebreaker "Almirante Irizar".  Samples were obtained underway by means of a centrifugal pump with the intake built into the ship's hull at approx. 9 m below sea level, filtered through a 26 um - mesh net, and preserved with 5% buffered formaldehyde. Sampling volumes varied around 1000 to 2000 liters (1 to 2 hours each). Biomass data were calculated from the settling volumes (24 hours). Diatoms were by far the most important component of all samples, both in terms of numbers and in terms of biomass, followed in considerably lower proportions by dinoflagellates, silicoflagellates, tintinnids, and several minor groups. Due to the sampling technique used, the zooplankton was almost exclusively represented by microplankters. Comparison of the wind-speed data at the times of the samplings with settling volumes of plankton (Fig. 1, bottom) yielded a very highly significant negative correlation (Fig. 2). Most previous reports indicate that, during the Antarctic summer, phytoplankton peaks are concentrated between the surface and approx. 50 to 100 m (El Sayed and Weber 1982; Schnack et al. 1985; Tilzer et al. 1985; Bodungen et al. 1986). At high latitudes of the Weddell Sea maximum phytoplanktonic concentrations have been recorded at 0 to 5 m below the surface (Bro"ckel 1985). Wind-induced turbulence and mixing can affect the upper 100 to 200 m of the water column (Sverdrup et al. 1942) - which is well in excess of the layer of highest phytoplanktonic abundance - thus disturbing normal stratification and diluting the surface and subsurface plankton-rich strata with deeper, more sparsely populated waters. It seems interesting to point out that similar phenomena have been reported for near-surface yields of planktonic abundances in connection with natural (wind-forced) and artificial disturbance of the vertical stratification (Boltovskoy at al. 1984; Hillman-Kitalong and Birkeland 1987; Boltovskoy and Mazzoni in press). Admittedly, the data base used has several drawbacks for the type of analysis presented in this paper. Most important among these are the method for estimating microplanktonic biomass values (i.e., settling volumes), and the mesh size of the net used, which underestimates the smallest phytoplanktonic components. Settling volumes have been shown to be virtually identical to dry-weight values, and to paralell very closely cell counts and biomass estimates based on cell volumes (Kopczynska and Ligowski 1982); thus, our data are most probably in very good agreement with estimates based on more precise methods. Due to the mesh size of the net used this biomass represents a very important fraction of the total microplanktonic biomass (all our samples were dominated by Corethron criophilum and by rather large and often chain-forming species of the genera Chaetoceros and Rhizosolenia), but it should be taken with caution if overall standing stock is sought. Our results therefore indicate that the relationship with wind force is valid within the size-range retrieved. On the other hand, unless smaller phytoplankters have a significantly different vertical distribution from that of the size-fraction collected, the influence of wind-force also affects the near-surface biomass of these smaller components. The correlation coefficient between wind-force and biomass (Fig. 2) indicates that the null hypothesis (i.e., lack of interdependence between the two variables) can be rejected with a very high degree of confidence, but it suggests that only about 20% of the observed variations in biomass can be explained by wind. It is probable that a more adequate data base would have shown that the actual relationship between the two parameters is considerably closer than this result; wind force during sampling is but a rough estimate of the real causes engendering water and, consequently, microplankton mixing: the fetch of wind (i.e., distance over which the wind has blown uninterrupted by land or ice), and its speed and constancy before sampling. Furthermore, the abundance of microplankton is not homogeneous throughout the areas covered (e.g., El-Sayed 1970; Estrada 1987; our data), which obviously strongly restricts the coherence of the correlation presented. Given the special characteristics of the open-ocean Antarctic and of operational conditions there (strong winds, none or few replicates for each measurement) the bias under discussion is probably of greater relevance in these environments than elsewhere.